1. Field of the Invention
The present invention relates to a directional control valve for controlling working cylinders or servomotors, in particular for controlling an actuating cylinder of a turbo-machine, such as a gas or steam turbine. A control piston (pilot piston) of the directional control valve is displaced using a force-regulated magnet and thus varying the flow cross-section between hydraulic connections and/or alternately producing connections between various connection pairs.
2. Description of the Related Art
Published application DE 28 30 332 describes an electrohydraulic valve unit having a proportional servovalve, a directional control valve coupled thereto for controlling a working cylinder, and an electromechanical transducer for generating a force which is linearly dependent on the electrical input signal for adjusting the servovalve. The directional control valve has a valve spindle and the valve spindle of the directional control valve is coupled by way of a spring to the valve spindle of the servovalve. On the other side of the valve spindle of the servovalve, there is a second spring, which acts in the opposite direction to the first spring, the spring constant of the second spring being greater than that of the first spring. The electromechanical transducer is implemented as an electromagnet.
The use of directional control valves for the control and/or position regulation of working cylinders is known. For single-acting cylinders, 3/3 directional control valves having a spring return are used and 4/3 directional control valves having a spring return are used for double-acting cylinders. These each have a control piston, which is actuated by way of an electromagnet. The electromagnet presses the control piston against a spring and thus connects a pressure connection P (reference characters are provided as a convenience in this section and while not referring to any of the drawings directly, they do provide a reference to similar connections of the present inventions) to a consumer connection and/or one of two consumer connections A, B, in order to conduct pressurized hydraulic medium by way of the particular connection into a predetermined chamber of the consumer. For example, in the case of double-acting working cylinders, the consumer connection A is connected to a first cylinder chamber of the working cylinder, and the consumer connection B is connected to a second cylinder chamber, which is separated by a cylinder piston from the first cylinder chamber. Depending on which of the cylinder chambers pressurized hydraulic medium is introduced into from a hydraulic pressure supply by way of a directional control valve, a piston rod attached to the piston of the working cylinder is retracted or extended.
In such double-acting cylinders, the 4/3 directional control valve is typically implemented in such a manner that whenever the first consumer connection A and thus the first cylinder chamber of the working cylinder is connected to the pressure connection P, the second consumer connection B and thus the second cylinder chamber of the working cylinder is connected to a tank connection T of the directional control valve or vice versa. The tank connection T is distinguished by a comparatively low hydraulic pressure, so that hydraulic medium slides out of the cylinder chamber connected to the tank connection T and is guided to a hydraulic tank.
Depending on whether, and how far, the control piston of the directional control valve is displaced using the electromagnet against the pressure force of the restoring spring, either the first consumer connection A is connected to the pressure connection P or the second consumer connection B is connected to the pressure connection P and the particular other consumer connection is connected to the tank connection T. Hydraulic medium thus either flows to one or the other side of the piston of the working cylinder and the retraction or extension of the piston and/or a piston rod of the working cylinder, which is attached to the piston, is controlled.
In order to position the piston with its rod at a position, a closed control circuit is required. For this purpose, a position measuring device, also referred to as a position encoder, is provided on the working cylinder, whose measuring signal is returned to control electronics integrated in the electromagnet of the directional control valve. These control electronics compare the measured value to the target value and calculate a new manipulated variable for the magnet from the difference. The force of the magnet then accordingly increases or decreases and displaces the control piston in the required direction in order to correct the location of the piston of the working cylinder. The control piston of the directional control valve is non-positively connected to the magnet armature of the electromagnet. The magnet armature is, the part of the magnet which is retracted or extended from the magnet by varying the control voltage or the control current strength, and the adjustment movement of the control piston is performed directly by the magnet armature, such that both components, the magnet armature and the control piston, always move jointly in the displacement direction of the control piston.
The flow cross-section between the consumer connection A, B and the pressure connection P and/or the tank connection T is determined by a ring gap, which the control piston delimits with a control housing, in which it is situated, so it is displaceable in the axial direction. Because the stroke of the electromagnet is limited, the flow cross-sections which a typical directional control valve can open and close are limited. The diameter of the control piston also cannot be enlarged arbitrarily in order to thus expand the flow cross-sections, because its mass thus increases and it could no longer be exactly dynamically positioned in connection with a spring in the closed control circuit by the magnet. In particular, the occurring mass forces, natural frequencies, friction forces, and oscillations are problematic.
To be able to use directional control valves for controlling actuating cylinders of turbo-machines in spite of the described problems, having a larger control piston diameter for enlarging the flow cross-sections in the directional control valves, on the one hand, there is the possibility of implementing the electromagnets used as larger and stronger. However, this results in higher inductances and larger armature masses, which has disadvantageous effects on the dynamic response, the required installation space, and the costs of the directional control valve.
In another design, additional position measuring devices and/or position encoders are provided for the measurement of the position of the control piston, and the measuring signal is returned to the control electronics of the electromagnet. An enlargement of the magnet is not necessary in these embodiments. However, the sensitivity of the position encoder with respect to temperature and oscillation influences is disadvantageous. In particular in the case of the use of such a directional control valve for controlling and/or regulating an actuating cylinder of steam turbines or gas turbines. The high ambient temperatures encountered by the position encoder, which are caused by the medium of gas or steam, and is varied with respect to its flow quantity, result in short maintenance intervals and early breakdown of the directional control valve.